Treatment of pain remains a major unmet medical need. Opiate drugs continue to be the most important therapies for the treatment of moderate to severe pain but suffer from severe side effects that diminish quality of life. It is known that neurons in the pain transmission and modulatory pathways undergo plasticity that results in amplification of pain signaling in settings of tissue injury (i.e. the conditions in which pain relieving drugs are used). Unexpectedly, such pronociceptive adaptations also occur as a consequence of repeated exposure to opiates. Increased signaling through the substance P-NK1 pathway strongly contributes to peripheral and central sensitization to enhance pain. This multidisciplinary proposal involving five different laboratories, is aimed at testing the hypothesis that pain relieving drugs can be designed with a priori consideration of mechanisms of amplification that diminish desired analgesia, i.e., drug design for disease. We hypothesize that drugs that have opioid agonist (mostly mu) and NK-1 antagonist activity within a single molecule would provide effective pain relief, particularly in conditions of chronic pain with diminished side effects allowing for greater tolerability and increased quality of life. Thus, we propose to exploit recent developments in our de novo design and pain-related pharmacology, physiology and molecular biology to develop new classes of peptidomimetic ligands with a profile of mu/delta opioid receptor agonist activities and neurokinin 1 antagonist activities. Our Specific Aims are: Aim 1: To design, synthesize and characterize novel metabolically stable and systemically active bifunctional preferring /4 opioid agonist ligands with NK-1 antagonist activities based on TY027, a current lead compound (delta opioid agonist preferring/NK1 antagonist). Biophysical studies (NMR, CD, PWR, etc.) in conjunction with computational chemistry will be used to evaluate 3-D conformations critical to bioactivity. Aim 2 will characterize the pharmacology of these compounds in vitro and in vivo for (a) analgesic/antihyperalgesic efficacy; (b) propensity for developing antinociceptive tolerance and opioid-induced hyperalgesia; (c) sedation; (d) gastrointestinal side effects; and (f) addictive liability. Aim 3 will evaluate the (a) metabolic stability of our novel compounds, (b) perform pharmacokinetic analysis, (c) evaluate BBB penetration and whether the compounds are substrates for P- glycoproteins, and (d) perform a broad screen for possible off-target activities. Our goal will be to discover a highly efficacious ligand, and backup ligand, that can be brought forward as a preclinical drug candidate during this grant period.